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b=Co09+6WRFlcwqsospnar4EGcys3raTpikkCYP/B/3HB4f81ufpt1wUl9dRZRvIZhq ir9DClZF0v02mvrLjG9ac+WLlrQLJ4v/Ei8y5zMBqTwZyMio8zGaL4DQ+7GCMKbNYP +m5AJzrCkD1hAIKSGungpJeChB2QdIrqVgvAvybVKHuwIYRe19veAKs+sA1zOFUnMD A8233TBM8kmwt1KsvmtXp8Y8SOtsARfV+MqjRICLQpPBKKuXxKXE9MB7e81VPPWguK JnGY8FJwND+KWuRYoChlKmFe4bS+T84Ec4sSLI+RBqKRoojRyIAsbWdwFmHNT7G44M CsbRtU5TnN0wg== Message-ID: <2fc85df3-cffc-4786-a7a8-0f0a6347fd59@kernel.org> Date: Sat, 14 Nov 2026 12:11:49 +0100 X-Archived-By: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/hmm: reject shadow-stack mappings To: "Gregory Price (Meta)" , Jason Gunthorpe Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@meta.com, leon@kernel.org, akpm@linux-foundation.org, kas@kernel.org, dave.hansen@linux.intel.com, bp@alien8.de, rppt@kernel.org, stable@vger.kernel.org, "Lorenzo Stoakes (ARM)" References: <20261113195536.2660763-1-gourry@gourry.net> <20261113200303.GA395967@nvidia.com> <179460249376.482102.14669935557901467762.b4-reply@b4> <20261114002027.GE3706007@nvidia.com> <179464112539.482102.8723246686117488051.b4-reply@b4> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 85D4F120005 X-Spam-Status: No, score=-7.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=4.0.1 X-Stat-Signature: kbsmmmxyi636di8ocd571nmx9ccbzqbd X-Rspam-User: X-HE-Tag: 1794654718-87673 X-HE-Meta: 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 kDGgb/Tb t259Cdv0HppGJWYUE3qitXLuesWYeUUxNkf8YNKAxUHtCxdEZDBSHpWnJ2hepPdUElBULe1URgSSddNfKGbSdvL9UbPAO8P4WvJpPvhjeO60yC1spvB5SQAV1F+6f4Z/UPy1oSNFITstFboE8uN/Q+egxZjQPXb4Jk3bEHNNJ6DWFRuZLclzTktdublJOVQfmP1azP2sQjgqK6T7CvvWkUQHLU6WIqcVX7RfV47B+Fi4JsNYkLcZHQ963BGo9Ik74kinjNsF8E97HL+9s22ZvrgKy79SHxEYVu0Nq Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On 11/14/26 08:25, Gregory Price (Meta) wrote: > On 2026-11-13 20:20 -0400, Jason Gunthorpe wrote: >> On Fri, Nov 13, 2026 at 03:41:33PM -0500, Gregory Price (Meta) wrote: >>> >>> Some of the context is here: >>> https://lore.kernel.org/all/20261111-mason-migrate-et-al-fixes-v1-10-3d2454362e8d@kernel.org/ >>> >>> We had a similar issue with migration, but this should handle the DMA >>> case as well. >>> >>> It's "technically writable" but the kernel is supposed to prevent that. >> >> Bah, it's not writable at all. The CPU PTE has write disabled and >> someone thought it was a good idea to make pte_write() return true >> because the CPU allows writes through a special instruction on this >> special PTE? So now all the places like this think its writable, when >> it really isn't. >> > > I completely missed this, that will teach me to trust pte_write() I > guess... what a bizarre design choice. I'm sure there's "reasons". > >> You could also fix this by having something like pte_real_write() or >> whatever that doesn't lie about the write bit in the PTE :| >> > > or rename pte_write() to pte_fake_write() instead and restore > pte_write() to its former truthful glory It's writable by the hardware, that's what matters to core-mm, for example, when it comes to things like fork(), mproetect() etc. *nobody* besides selected core-mm code should ever write to random PTEs. They shouldn't ever even care about pte_write(). We have GUP for that, which takes care of that oddity properly when deciding whether we can *actually* write to a mapped page. Well, and we have HMM that reimplements part of GUP in ugly ways (sorry) that we missed. -- Cheers, David